Ink-jet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper, transparency film, or textiles. Low cost and high quality of the output, combined with relatively noise-free operation, have made ink-jet printers a popular alternative to other types of printers used with computers. Essentially, ink-jet printing involves the ejection of fine droplets of ink onto print media in response to electrical signals generated by a microprocessor.
There are two basic means currently available for achieving ink droplet ejection in ink-jet printing: thermally and piezoelectrically. In thermal ink-jet printing, the energy for drop ejection is generated by electrically-heated resistor elements, which heat up rapidly in response to electrical signals from a microprocessor to create a vapor bubble, resulting in the expulsion of ink through nozzles associated with the resistor elements. In piezoelectric ink-jet printing, the ink droplets are ejected due to the vibrations of piezoelectric crystals, again, in response to electrical signals generated by the microprocessor. The ejection of ink drop-lets in a particular order forms alphanumeric characters, area fills, and other patterns on the print medium.
Inks useful in ink-jet printing are typically composed of either pigments or dyes. Pigments are very small insoluble solid colorant particles wherein the molecules are strongly associated with one another through intermolecular forces such as Van der Waals attraction, pi-pi interactions or hydrogen bonding. These forces of attraction prevent solvation of the molecules by common solvents such that they cannot form solutions like their dye counter-parts. Conventionally, pigments are used in printing inks by suspending them in a liquid medium using a high-energy dispersion process with the aid of dispersing agents. One disadvantage of using pigments is that their dispersions are inherently thermodynamically unstable and eventually the pigment particles agglomerate to larger particles that tend to settle. Moreover, pigments lead to the inks taking a relatively long time to dry on the media, which in turn leads to increased opportunity for the ink to run or smear when subjected to moisture or water; the degree of water resistance is called "waterfastness". Further, the tendency of one color of ink to run into the adjacent color is know at "bleed". This is most apparent with black ink bleeding into color inks. Several methods have been employed by ink formulators to achieve bleed control to varying degrees of success. Many pigmented inks possess one or more of the foregoing properties. However, few ink compositions possess all of those properties, since an improvement in one property often results in the degradation of another. Thus, inks used commercially represent a compromise in an attempt to achieve a pigmented ink evidencing at least an adequate dispersibility and performance in each of the aforementioned properties.
Accordingly, investigations continue into developing ink formulations that have both good dispersibility and improved properties such as improved bleed control and good waterfastness all without sacrificing performance in other necessary properties.